Many situations require measuring sounds having a high Sound Pressure Level (SPL), such as exceeding 130 or even 140 dB. The level of 140 dB can cause damage to human ears and therefore shall be monitored in working environments.
Sound dosimetry measurements can be performed using acoustic dosimeters. An exemplary dosimeter has been described in a US patent U.S. Pat. No. 7,913,565, which discloses a dosimeter comprising an electronic circuit for receiving at least one signal representing a hazardous level, equipped with a sensor, for example a microphone, and a processor for determining an accumulated dose in a specific measurement window.
The acoustic dosimeters which are now commercially available typically use capacitor microphones. The capacitor microphones provide good measurement parameters, but are relatively expensive. Moreover, they are sensitive to mechanical shocks and can be easily damaged, for example when dropped on a hard surface.
There are known MEMS microphones (MicroElectroMechanical Systems). MEMS microphones have a number of advantages, such as high resistance to mechanical impacts, small dimensions and low price. However, MEMS microphones have a relatively small dynamic range of measurement and are typically limited to measuring sound levels not exceeding 130 dB. Therefore, MEMS microphones cannot be directly used in acoustic dosimetry applications which require measuring sound levels higher than 140 dB SPL peak.
It is known that the upper measurement limit of the microphone can be raised by coupling the microphone with an external attenuator, to lower the acoustic pressure reaching the microphone membrane. The measurement limit of the microphone is therefore increased by the value of attenuation of the attenuator. However the method for making such attenuator for the wide frequency measurement range is not known. MEMS microphones have not been used so far in applications requiring sound level measurement higher than their capabilities, no external attenuator for MEMS microphone has been developed yet.
A European patent application EP2592844A1 discloses a microphone unit that includes a MEMS microphone within an enclosure that forms a first sound guide space and a second sound guide space separated by the diaphragm of the MEMS microphone from the first sound guide space. Therefore, the MEMS microphone is configured as a differential microphone. The unit is not particularly configured to attenuate sound level reaching the MEMS microphone to enable measurement of sound level higher than the capabilities of the MEMS microphone.
Therefore, there is a need to develop a device for measuring sound level using a MEMS microphone, with a sound measurement limit higher than the basic measurement limit of the MEMS microphone.